Title

Author

Date

May 1986

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Materials Science and Engineering

Institution

Oregon Graduate Center

Abstract

Barium titanate has a wide variety of applications, due to its ferroelectric, dielectric and piezoelectric properties. Some of these applications are as transducers, memory, delay line and capacitors. Due to its high dielectric constant barium titanate is by far the most widely used material for multi layer ceramic capacitors. Degradation is still a common malady in this material even after the many reported studies. A majority of the previous studies were not carried out on thermodynamically defined samples. In this investigation thermodynamically well defined and high purity BaTiO3 with various Ba/Ti ratios and BaNi[subscript x] Ti[subscript 1-x]O3 with x = 0.005 to x = 20 at. % were prepared by the liquid mix technique. The observed X-ray diffraction pattern for BaTiO3 showed very little change in lattice parameter with Ba/Ti ratio whereas Raman spectroscopy was very sensitive to these small changes from the stoichiometry. Nickel solubility was estimated to be 1.5 at. % Ni in BaTiO3 at 1275Â° C. The solubility of nickel was also found to be a function of sintering temperature where higher temperatures led to lower solubility. By X-ray diffraction and Raman methods it was shown that the second phase formed at 1.5 at. % Ni doping is a hexagonal BaTiO3. The Raman method has shown that this hexagonal phase has a different lattice dynamics than the hexagonal phase which is formed in pure BaTiO3, though X-ray patterns are the same for both these phases. Temperature dependent Raman spectra showed that the room temperature Raman spectrum of barium titanate is a combination of first order and second order scattering processes. Electrical conductivity measurements between 200 and 550Â° C have shown that in this temperature range, the conduction was by a band process and it was predominantly electronic in nature. The band gap for undoped stoichiometric barium titanate was found to be 2.74 Â± 0.05 eV. The electrical conductivity with the different Ba/Ti ratios and various amounts of Ni doping was measured between 850 and 1100Â° C and between 10Â° to 10[superscript -22] atm. of oxygen. The. effect of nickel doping on the conductivity was observed for small doping (0.0005 at. %). For the 0.01 at % nickel doped sample the conductivity was proportional to P [subscript O2] [superscript - Â¼] between P [subscript O2] [superscript 10 - 8] and 10[superscript -22] atm. indicating the dominance of the acceptor dopants. As the nickel increased the minimum in the conductivity and thus the n to p transition was shifted to a lower partial pressure of oxygen and the p-type conductivity increased. For the 1 at. % nickel doped sample electronic compensation was observed. At 900Â° C and 10[superscript -13] atm. of P [subscript O2], nickel changes valency from +3 to +2. Nickel substituted on the titanium site in the barium titanate lattice with +3 valence. For BaTiO3, the data were found to be proportional to -1/6 power of oxygen partial pressure for the pressure range 10 [superscript -8] to 10[superscript -22] atm., and proportional to P [subscript O2 [superscript +Â¼] for the oxygen pressure range > 10[superscript -4] atm. The deviation from the ideal Ba/Ti ratio was found to be accommodated by neutral vacancy pairs. Pure BaTiO3 with various Ba/Ti ratios showed little degradation at 10 kv /cm. stress at 250Â° C, but as the Ni increased the resistance to degradation decreased. Resistance to degradation was found to be sensitive to the Ba/ (Ti + Ni) ratio. When this ratio was greater than unity the samples have high degradation resistance. This study did not show any correlation between the degradation and the migration of oxygen vacancies towards the cathode. Microscopic observation did not yield information about the second phase, (10 to 20 at. % nickel doped samples) mainly due to the poor contrast between the two phases, although two different methods of etching were employed. The Raman technique however showed promise that it can make in situ structure and phase determination nondestructively. In the Raman spectra of reduced barium titanate an extra band at 820 cm [superscript -1] has been observed. This band was correlated .to the presence of oxygen vacancies.

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